Process for the purification of an oxidation off gas with energy recovery

ABSTRACT

In the purification of an off-gas stream originating from a process of oxidizing a liquid mixture of para-xylene with methyl para-toluate with an oxygen-containing gas under pressure, the off-gas being contaminated with at least one of an aliphatic and an aromatic impurity, which comprises expanding the off-gas stream in an expansion turbine for generating mechanical or electrical energy so as to obtain an at least partial recovery of the energy contained in the off-gas, the improvement comprising subjecting the off-gas to at least one absorption step under pressure to remove at least one component from the off-gas and subjecting the resultant off-gas stream to combustion under a pressure of more than 3 bar prior to expanding the off-gas stream in the expansion turbine.

This application is a continuation, of application Ser. No. 08/027,161, filed Feb. 24, 1993.

TECHNICAL FIELD

The invention relates to a process for the purification of an off gas which originates from a reaction process operated under pressure and in particular from an oxidation process operated using oxygen-containing gas and which is contaminated with aliphatic and/or aromatic substances, with at least partial recovery of the energy contained in the off gas, in which process the off gas stream is expanded in an expansion turbine for generating mechanical or electrical energy. The off gas to be purified originates in particular from the oxidation of para-xylene (p-X) with air by the so-called Witten DMT process for the preparation of dimethyl terephthalate (DMT).

In the Witten DMT process, a mixture of p-X and methyl para-toluate (p-TE) in the liquid phase is oxidized in the absence of solvents foreign to the process and of halogen compounds at a pressure of about 4 to 8 bar and a temperature of about 140° to 180° C. with air (atmospheric oxygen) in the presence of dissolved heavy metal oxidation catalysts, for example in the presence of a mixture of cobalt and manganese compounds (cf. German Patent 20 10 137).

In this process carried out industrially on a large scale, the oxidation with air produces an off gas which, depending on the oxidation pressure and on the temperature and the physical properties, is typically saturated with the following fractions:

a) 0.4 to 0.6% by weight of a relatively high-boiling fraction of DMT, monomethyl terephthalate (MMT), terephthalaldehydic acid methyl ester (TAE), para-toluic acid (p-TA) and terephthalic acid (TPA). Some of these products form during the cooling off sublimates having a high melting point, unless they are dissolved in the other components;

b) 14 to 22% by weight of a water-insoluble fraction having a medium boiling point and consisting of products such as p-TE, methyl benzoate (BME) and p-X;

c) 7 to 10% by weight of a fraction having a medium boiling point and consisting of water-soluble products such as acetic acid, formic acid, water of reaction and methanol;

d) 0.2 to 0.3% by weight of low boilers, such as methyl acetate, methyl formate, acetaldehyde, formaldehyde, dimethyl ether and the like.

The carrier off gas for the above substances consists of:

e) the nitrogen of the air;

f) a residual oxygen content of 0.5 to 3% by weight (of 6 to 8% by weight when the explosion limits are reached);

g) the byproducts of the reaction: 1 to 3% by weight of CO₂ and 0.3 to 2.0% by weight of CO.

From the economic point of view, the above-mentioned substances are divided into so-called useful products and waste substances. The useful products are recycled to the total process.

The waste substances must be disposed off in a manner which is as environment-friendly as possible. These waste substances include CO and the low boilers acetaldehyde, formaldehyde and dimethyl ether, and acetic acid and formic acid.

In the case of the useful products, a distinction can be made between water-soluble products, such as methanol, and water-in-soluble products, such as p-X, DMT, p-TE, etc.

BACKGROUND ART

Regarding the "Witten DMT process" (Katzschmann process), reference may be made to German Patent 10 41 945, German Patent 12 99 627, DE-A-20 14 012, DE-C3-22 44 662, DE-C3-24 27 875, DE-C2-29 16 197, DE-C2-30 11 858, DE-C2-30 37 045, DE-C2-30 44 617, EP-B1-0 053 241, DE-C1-31 51 383, EP-B1-0 156 133, EP-B1-0 157 122, DE-C1-34 40 650 and WO 90/909367 and Hydrocarbon Processing, November 1983, page 91. Where reference is made in the present description to the total process, these publications are referred to.

The off gas from the oxidation by the Witten DMT process is usually cooled in several stages, the relatively high-boiling and medium-boiling components substantially condensing stepwise. After scrubbing with cold water, the off gas stream is substantially freed from residual organic constituents in an activated carbon adsorption (Ullmann, 4th Edition 1982, Volume 22, page 529 et seq.; DE-C1-34 40 650 and DE-A1-24 61 092).

A similar waste gas purification with cooling and subsequent scrubbing with water is known for the oxidation waste gases in the so-called AMOCO process (Ullmann, 4th Edition 1982, Volume 22, page 519 et seq.).

In order to utilize the stored mechanical energy, it is usual to use the off gases, which are under a pressure of 5 to 15 bar, in an expansion turbine for driving a compressor for inlet air.

This off gas purification has the disadvantage that the energies contained in the off gas can be utilized only to a limited extent. Finally, it is not possible to remove the CO present in the off gas by means of desorption using activated carbon. Another disadvantage is that the adsorption stage (activated carbon plant) can only be operated discontinuously and that working up of the activated carbon desorbate is expensive.

GB-A-2 097 476 (=DE-A1-32 15 437) discloses a process for utilizing the mechanical energy of a residue gas, in which the residue gas is used for cooling a gas turbine with subsequent mixing the residue gas with the combustion off gas. Combustion of the residue gas is neither intended nor possible in this process.

EP-A- 0 222 158 discloses a process for removing organic impurities from a gas. Preferably, the impurities are condensed in a first stage of purification and the remaining impurities are burnt in a second purification stage. The energy gained from this second stage of purification for inst. can be utilised for the generation of current. EP-A 0 222 158, however, does not make reference to the purification of an off-gas from a reaction process operated under pressure.

DE-A- 27 59 208 discloses the utilization of solvent vapours in exhaust air for operating combustion aggregates. At this the off-gas loaded with organic solvents can be burnt. The units for thermal burning of solvents can be utilised for heat generation. Furthermore it is possible to perform a conversion into mechanical energy. DE-A- 27 59 208 also does not disclose the purification of an off-gas from a reaction process operated under pressure.

DISCLOSURE OF INVENTION

The object of the present invention is therefore to utilize as substantially as possible the energy contained in the off gas (pressure, temperature, oxidizable products). A further object of the invention is to remove as substantially as possible, in continuously operated stages, the organic products present in the off gas. Finally, it is a further object of the invention to minimize the CO content of the off gas.

The invention achieves this object by virtue of the fact that the off gas is burned under a pressure of more than 3 bar and the resulting flue gases are used for generating mechanical or electrical energy in an expansion turbine.

Provided that no additional compression is envisaged, the pressure in the combustion corresponds approximately to the pressure in the oxidation, that is to say is in general more than 4 bar and preferably 5 to 50 bar.

In a preferred application of the process according to the invention for the purification of the oxidation off gas from an oxidation of p-X with air with energy recovery, the combustion can preferably be preceded by a (partial) condensation, in particular at a temperature of 35° to 60° C., preferably at about 40° C. By separating off the condensate, the major part of the medium- and high-boiling organic constituents of the off gas (p-TE, DMT, BME) and of the p-xylene and of methanol can be removed from the off gas. The (partial) condensation can also be carried out in a condensation scrubbing stage.

The remaining off gas stream contains predominantly p-X and methanol as useful products. These substances are preferably removed in an absorption, the absorption preferably being carried out in several stages. In the first absorption stage, the absorbent (solvent) consists predominantly of p-TE. The p-X of the off gas is dissolved in the p-TE, with the exception of traces, and is washed out with the p-TE. In a preferred embodiment of the invention, the off gas freed from the p-X and enriched with the solvent (p-TE) of the first absorption stage is washed, in a second absorption stage, with a further, preferably lower-boiling solvent, which consists in particular predominantly of BME and/or methanol, by a countercurrent method The off gas, which preferably cools to 20° to 30° C., is freed in this second absorption stage in particular from the solvent of the first stage (p-TE) and then contains virtually exclusively methanol as the useful product.

The methanol still contained in the off gas is preferably absorbed by further scrubbing with water, the temperature being about 10° to 20° C., preferably about 15° C., for reasons relating to energy. After this third absorption stage, the off gas contains only traces of useful products.

For the process according to the invention, it is particularly advantageous to choose, as the absorbent for methanol, a process waste water which is enriched with distillable waste substances (for example acetic acid, formic acid, formaldehyde, etc.) but is free of useful substances. The vaporous fraction of the waste water, which fraction is taken over in the off gas stream, can then be disposed off very advantageously by subsequent combustion. Even if methanol absorption is unnecessary, it is very advantageous if the off gas which has been freed beforehand by the stated condensation of useful products and at the same time completely or partly dried is saturated, by means of waste water vapours produced by waste heat, with the distillable waste substances present therein.

The combustible constituents still present, in particular CO and some organic products originating from the scrubbing water, such as acetic acid, formic acid and the constituents of the off gas which have not been removed by absorption, are used, according to the invention, for heating the off gas by continuous combustion under a pressure of more than 3 bar, the mechanical or thermal energy being used in an expansion turbine. Preferably, oxygen (air) and a fuel are additionally fed to the off gas prior to combustion, in order to achieve energetically useful utilization with relatively high efficiency. However, it is also possible to burn the oxidizable constituents catalytically under pressure.

In a preferred embodiment of the invention, after the final absorption stage the off gas is first heated with the oxidation off gas by a countercurrent method and then saturated with a process waste water which is contaminated with organic waste substances. The waste water can be evaporated with waste heat from the process. In this way, some of the process waste water can be disposed off in a manner which is energetically particularly advantageous.

Using the process according to the invention, it is possible, in a surprisingly simple manner, to achieve complete purification of the off gas from oxidation processes operated with oxygen-containing gas, and the energy present in the off gas (thermal, material and mechanical) can be substantially recovered.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to an embodiment and to the drawing. As diagrams of the plant,

FIG. 1 shows an off gas purification plant according to the invention

FIG. 2 shows a downstream energy recovery plant

FIG. 3 shows an alternative off gas purification plant similar to FIG. 1.

The mass streams 100 to 242 (circled) shown in FIG. 1 and 2 are specified in Table 1. In Table 2, the energy streams 200 to 232 (designated with a rectangular border in the Figure) are balanced.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

In a plant for the production of terephthalic acid (TPA) from DMT as an intermediate product, corresponding to DE-C2-3 044 617, having a capacity of 250,000 tonnes per year, about 93,000 kg/h of off gases are obtained from the oxidation and are purified in an off gas treatment stage according to FIG. 1 and 2.

In addition to the oxidation off gas, the following mass streams are taken from the total process:

Mass stream 108, a fraction, containing predominantly p-TE, from the distillation of the crude ester;

Mass stream 114, a fraction, containing predominantly BME, from the so-called working up of the B-ester;

Mass stream 184, a process water contaminated with organic waste products;

Energy stream 222, about 600 kg/h of dimethyl ether (DME), which is obtained by distillation from the methanol-containing top product of the hydrolysis;

Energy stream 224, about 858 kg/h of combustible liquid residues from working up of the residue;

Energy stream 226, about 760 kg/h of natural gas.

The off gas stream 100 from the p-X oxidation, at a temperature of 158° C. and a pressure of 7 bar and saturated with the substances according to Table 1, is cooled via the heat exchanger 2 to a temperature of 120° C. at about the same pressure with the off gas stream 128 to be removed, by a countercurrent method (FIG. 1).

Thereafter, the cooled off gas stream 101 containing the simultaneously predominantly condensed relatively high-boiling components of the contaminating substances is fed to the condenser 4 operated with cooling water, for further cooling and condensation of the residual relatively high-boiling components and some of the medium-boiling components of the useful products and of the water.

The resulting liquid phase 102 is removed from the condenser 4 at a temperature of 40° C. and expanded into a separator (not shown).

The off gas mixture 104 emerging from the condenser 4 is fed, at a temperature of 40° C. and with a load of 1.25% by weight of p-X, 0.16% by weight of methanol and 0.17% by weight of low boilers, to the bottom of the first absorption stage of the two-stage absorption scrubbing column 3 for recovering the p-xylene.

p-TE fraction 108 is taken, at a temperature of 140° C., from the p-TE fraction which is obtained in the total process from the separation of the crude ester by distillation and is recycled to the oxidation (not shown), and said fraction 108 is brought to the pressure of 7 bar by means of the pressure-increasing pump 6, cooled to a temperature of 100° C. by means of the heat exchanger 8 operated with condensate and fed as solvent 110 to the top of the first absorption stage of the two-stage absorption scrubbing column 3 for scrubbing the off gas mixture 104 flowing through, by a countercurrent method.

The off gas mixture which is obtained at the top of the first absorption stage at a temperature of 60° C., has a low xylene content and is enriched with p-TE flows directly into the bottom of the second stage. In this second stage, the off gas stream is scrubbed for recovery of the p-TE and further cooled.

The BME fraction 114 originates in the total process from the separation of the so-called B ester fraction by distillation, said fraction mainly consisting of p-TE, BME, TAE and methanol. The B ester fraction is obtained from the fraction resulting at the top of the crude ester rectification and mainly consisting of p-TE. The BME fraction 114 has a temperature of 104° C. and is brought to the pressure of 7 bar by means of the pressure-increasing pump 10 and is fed into the outer cooling circulation of the second stage of the two-stage absorption scrubbing column 3, which circulation is operated by means of circulating pump 9, and is mixed, at a temperature of 46° C., with the liquid phase 112 obtained at the bottom of the second stage, a temperature of 46.5° C. (mass stream 116) being established. The circulating absorbent predominantly consisting of BME and p-TE is then cooled to a temperature of 25° C. via the heat exchanger 11 operated with cold water and is fed countercurrently, as coolant and solvent 118, to the top of the second stage for scrubbing, for further cooling and condensation of the useful products of the off gas mixture 104 flowing through.

The solvents which have collected at the bottom of the second stage and contain the recovered useful products consist mainly of p-TE and partly of methanol and BME, flow in a small amount freely into the first absorption stage and are removed at a temperature of 48° C., together with the p-X-containing p-TE fraction 106 of the two-stage absorption column 3, said fraction being obtained at the bottom, and is expanded into the storage tank for the p-TE fraction of the total process which is to be recycled to the oxidation.

The off gas mixture 120 emerging at the top of the two-stage absorption scrubbing column 3 and at a temperature of 26° C. and having a load of 0.12% by weight of methanol and 0.14% by weight of low boilers is fed to the bottom of the absorption column 13 for the recovery of the methanol.

The process waste water 184 having a low methanol content and at a temperature of 40° C. is brought to the pressure of 7 bar by means of the pressure-increasing pump 15, cooled to a temperature of 15° C. by means of the heat exchanger 17 operated with cold water and fed, as solvent 126, to the top of the absorption column 13 for scrubbing countercurrent to the off gas mixture 120 flowing through.

The methanol/water mixture 122 obtained at the bottom at a temperature of 20° C. is expanded into a separator (not shown).

The off gas mixture 128 emerging at the top of the absorption column 13 at a temperature of 15° C., freed from useful products and having a residual load of 0.28% by weight of low boilers is preheated to a temperature of 131° C. (mass stream 130) by heat released from the off gas stream 100 via the heat exchanger 2 by a countercurrent method and is then heated to a temperature of 340° C. (mass stream 230), together with the air 242 required for post-combustion, via the downstream heat exchanger 48 (FIG. 2) by means of the flue gas stream 236 to be removed and disposed of, by a countercurrent method. The heated off gas 230 is completely burned or incinerated in the combustion chamber 50 with the supply of fuel in the form of vapour and liquid residues obtained under pressure and consisting of dimethyl ether (DME) (energy stream 222) and DMT residue (energy stream 224) from the process and with natural gas (energy stream 226) as additional fuel, at a pressure of 7 bar and a temperature of 1000° C., with utilization of the heat of combustion present in the off gas stream 130.

The flue gas 232 obtained is cooled to a temperature of 650° C. (mass stream 234) via the heat exchanger 52, by heating up a heating medium and/or generating high-pressure steam, which are required in the process, and fed to the hot gas expansion turbine 54.

In the turbine 54, expansion of the flue gas to the pressure of 1.05 bar and at the same time cooling to a temperature of 380° C. take place.

The mechanical energy liberated is fed directly as drive energy to the air compressor 56 which runs on the same shaft, produces the air stream 150 for the oxidation at a pressure of 9 bar and at a temperature of 115° C. and supplies the air 242 for the post-combustion of the simultaneously liberated off gas mixture 130 under a pressure of 7 bar at a temperature of 75° C.

The water 152 obtained in the intermediate cooling during the air compression by the heat exchanger 27 operated with cooling water is removed.

The expanded and cooled flue gas 236 from the hot gas turbine 54 is cooled to a temperature of 165° C. for further utilization of the waste heat via the heat exchanger 48 by a countercurrent method with the gas stream 228 intended for post-combustion and is released into the atmosphere.

As can be seen from mass stream 238 in Table 1, the purified off gas contains virtually no more organic constituents and CO and can be safely emitted.

According to Table 2, a total of 10 kW of electrical energy and 18.81 MW of fuels are required for the off gas purification (energy streams 202, 206, 208, 212, 222-226). On the other hand, 11.1 MW are used for steam generation and 8.672 MW for compression of the oxidizer inlet air (mass streams 204, 228 and 230). A balance for the plant according to FIG. 1 and 2 gives a net energy recovery of 980 kW from the off gas.

A conventional off gas purification with three-stage condensation and adsorption on activated carbon has a net energy requirement of 1.852 MW without the associated working-up stages and the purification of the off gas is considerably poorer. Thus, the process according to the invention permits an energy saving of about 2.8 MW in addition to better purification.

EXAMPLE 2

FIG. 3 shows an alternative off gas purification which substantially corresponds to the plant shown in FIG. 1. After partial condensation of the contaminating substances in the condenser 4, the still dissolved p-X is removed from the off gas, initially in scrubber 3A with a mixture of p-TE and BME. In the second desorption 3B, the BME dissolved in the off gas stream and the p-TE are washed out with methanol 134. Finally, the methanol dissolved in the off gas is washed out in the scrubbing column 18 with water.

                                      TABLE 1.1                                    __________________________________________________________________________     Mass stream                                                                    Medium   Unit  100 101 102 104 106 108 110                                     __________________________________________________________________________     Amount   [kg/h]                                                                               93402                                                                              93402                                                                              24083                                                                              69319                                                                              6287                                                                               5000                                                                               5000                                    Pressure [bar] 7   7   7   7   7   1,1 7                                       Temperature                                                                             [°C.]                                                                         158 120 40  40  48  140 100                                     Heat content*                                                                           [kW]  10916                                                                              8365                                                                               638 569 167 362 259                                     High boilers                                                                            [% by wt.]                                                                           traces                                                                             traces                                                                             traces                                                  DMT DMI DMO                                                                             [% by wt.]                                                                           0,21                                                                               0,21                                                                               0,81    11,93                                                                              15,00                                                                              15,00                                   MMT      [% by wt.]                                                                           0,02                                                                               0,02                                                                               0,08                                                    TPA      [% by wt.]                                                                           traces                                                                             traces                                                                             traces                                                  TAE      [% by wt.]                                                                           0,03                                                                               0,03                                                                               0,12    3,18                                                                               4,00                                                                               4,00                                    p-TE     [% by wt.]                                                                           3,67                                                                               3,67                                                                               14,22                                                                              traces                                                                             58,56                                                                              73,60                                                                              73,60                                   p-TA     [% by wt.]                                                                           0,21                                                                               0,21                                                                               0,81    0,32                                                                               0,40                                                                               0,40                                    BME      [% by wt.]                                                                           2,10                                                                               2,10                                                                               8,12                                                                               traces                                                                             6,99                                                                               7,00                                                                               7,00                                    Xylenes  [% by wt.]                                                                           12,35                                                                              12,35                                                                              44,34                                                                              1,25                                                                               13,72                                           Methyl acetate                                                                          [% by wt.]                                                                           0,11                                                                               0,11                                                                               0,25                                                                               0,06                                                                               0,15                                            Methyl formate                                                                          [% by wt.]                                                                           0,05                                                                               0,05                                                                               0,08                                                                               0,04                                                                               0,02                                            Acetic acid                                                                             [% by wt.]                                                                           0,42                                                                               0,42                                                                               1,59                                                                               0,01                                                                               0,15                                            DME      [% by wt.]                                                            Formic acid                                                                             [% by wt.]                                                                           0,21                                                                               0,21                                                                               0,78                                                                               0,01                                                                               0,09                                            Acetaldehyde                                                                            [% by wt.]                                                                           0,05                                                                               0,05                                                                               0,06                                                                               0,04                                                                               traces                                          Methanol [% by wt.]                                                                           0,65                                                                               0,65                                                                               2,06                                                                               0,16                                                                               0,50                                            Carbon dioxide                                                                          [% by wt.]                                                                           1,50                                                                               1,50    2,02                                                Oxygen   [% by wt.]                                                                           2,32                                                                               2,32    3,13                                                Carbon oxide                                                                            [% by wt.]                                                                           0,41                                                                               0,41    0,55                                                Nitrogen [% by wt.]                                                                           68,48                                                                              68,48   92,27                                               Water    [% by wt.]                                                                           7,20                                                                               7,20                                                                               26,67                                                                              0,45                                                                               4,38                                            NO.sub.x as NO.sub.2                                                                    [% by wt.]                                                            __________________________________________________________________________      Comment: traces <0,01% by weight                                               *Reference temperature 0° C.                                      

                                      TABLE 1.2                                    __________________________________________________________________________     Mass stream                                                                    Medium   Unit  112 114 116 118 120 122 126                                     __________________________________________________________________________     Amount   [kg/h]                                                                               76833                                                                              100 76933                                                                              76933                                                                              68132                                                                              9392                                                                               9500                                    Pressure [bar] 7   1,1 7   7   7   7   7                                       Temperature                                                                             [°C.]                                                                         46  104 46,5                                                                               25  26  20  15                                      Heat content*                                                                           [kW]  2041                                                                               6   2290                                                                               1758                                                                               540 205 162                                     High boilers                                                                            [% by wt.]                                                            DMT DMI DMO                                                                             [% by wt.]                                                                           0,32    0,32                                                                               0,32                                                MMT      [% by wt.]                                                            TPA      [% by wt.]                                                            TAE      [% by wt.]                                                                           0,26    0,26                                                                               0,26                                                p-TE     [% by wt.]                                                                           37,83   37,78                                                                              37,78                                                                              traces                                          p-TA     [% by wt.]                                                                           0,01    0,01                                                                               0,01                                                BME      [% by wt.]                                                                           57,90                                                                              98,00                                                                              57,95                                                                              57,95                                                                              0,02                                            Xylenes  [% by wt.]                                                                           0,11    0,11                                                                               0,11                                                                               traces  traces                                  Methyl acetate                                                                          [% by wt.]                                                                           0,25    0,25                                                                               0,25                                                                               0,05                                            Methyl formate                                                                          [% by wt.]                                                                           0,07    0,07                                                                               0,07                                                                               0,04                                            Acetic acid                                                                             [% by wt.]                                                                           0,01    0,01                                                                               0,01                                                                               traces                                                                             1,55                                                                               2,22                                    DME      [% by wt.]                                                            Formic acid                                                                             [% by wt.]                                                                           0,08    0,08                                                                               0,08                                                                               traces                                                                             0,78                                                                               1,10                                    Acetaldehyde                                                                            [% by wt.]                                                                           0,05    0,05                                                                               0,05                                                                               0,05                                            Methanol [% by wt.]                                                                           0,83                                                                               2,00                                                                               0,83                                                                               0,83                                                                               0,11                                                                               0,86                                                                               0,05                                    Carbon dioxide                                                                          [% by wt.]            2,05                                            Oxygen   [% by wt.]            3,18                                            Carbon oxide                                                                            [% by wt.]            0,56                                            Nitrogen [% by wt.]            93,88                                           Water    [% by wt.]                                                                           2,28    2,28                                                                               2,28                                                                               0,05                                                                               96,81                                                                              96,62                                   NO.sub.x as NO.sub.2                                                                    [% by wt.]                                                            __________________________________________________________________________      Comment: traces <0,01% by weight                                               *Reference temperature 0° C.                                      

                                      TABLE 1.3                                    __________________________________________________________________________     Mass stream                                                                    Medium   Unit  128 130 150 152 184 228 230                                     __________________________________________________________________________     Amount   [kg/h]                                                                               68240                                                                              68240                                                                              83535                                                                              443 9500                                                                               95150                                                                              95150                                   Pressure [bar] 7   7   9   1,1 1,1 7   7                                       Temperature                                                                             [°C.]                                                                         15  131 115 25  40  123 340                                     Heat content*                                                                           [kW]  497 2761                                                                               2750                                                                               13  440 3329                                                                               9750                                    High boilers                                                                            [% by wt.]                                                            DMT DMI DMO                                                                             [% by wt.]                                                            MMT      [% by wt.]                                                            TPA      [% by wt.]                                                            TAE      [% by wt.]                                                            p-TE     [% by wt.]                                                                           traces                                                                             traces          traces                                                                             traces                                  p-TA     [% by wt.]                                                            BME      [% by wt.]                                                                           0,02                                                                               0,02            0,01                                                                               0,01                                    Xylenes  [% by wt.]                                                                           traces                                                                             traces      traces                                                                             traces                                                                             traces                                  Methyl acetate                                                                          [% by wt.]                                                                           0,05                                                                               0,05            0,04                                                                               0,04                                    Methyl formate                                                                          [% by wt.]                                                                           0,03                                                                               0,03            0,03                                                                               0,03                                    Acetic acid                                                                             [% by wt.]                                                                           0,10                                                                               0,10        2,22                                                                               0,07                                                                               0,07                                    DME      [% by wt.]                                                            Formic acid                                                                             [% by wt.]                                                                           0,05                                                                               0,05        1,10                                                                               0,03                                                                               0,03                                    Acetaldehyde                                                                            [% by wt.]                                                                           0,05                                                                               0,05            0,03                                                                               0,03                                    Methanol [% by wt.]                                                                           traces                                                                             traces      0,05                                                                               traces                                                                             traces                                  Carbon dioxide                                                                          [% by wt.]                                                                           2,05                                                                               2,05            1,47                                                                               1,47                                    Oxygen   [% by wt.]                                                                           3,17                                                                               3,17                                                                               23,13       8,79                                                                               8,79                                    Carbon oxide                                                                            [% by wt.]                                                                           0,56                                                                               0,56            0,40                                                                               0,40                                    Nitrogen [% by wt.]                                                                           93,73                                                                              93,73                                                                              76,57       88,80                                                                              88,80                                   Water    [% by wt.]                                                                           0,17                                                                               0,17                                                                               0,30                                                                               100,00                                                                             96,62                                                                              0,32                                                                               0,32                                    NO.sub.x as NO.sub.2                                                                    [% by wt.]                                                            __________________________________________________________________________      Comment: traces <0,01% by weight                                               *Reference temperature 0° C.                                      

                                      TABLE 1.4                                    __________________________________________________________________________     Mass Stream                                                                    Medium   Unit  232 234 236 238 240 242                                         __________________________________________________________________________     Amount   [kg/h]                                                                               97368                                                                              97368                                                                              97368                                                                              97368                                                                              110888                                                                             26910                                       Pressure [bar] 7   7   1,05                                                                               1,05                                                                               1,1 7                                           Temperature                                                                             [°C.]                                                                         1000                                                                               650 380 165 15  75                                          Heat content*                                                                           [kW]  30675                                                                              19650                                                                              10978                                                                              4517                                                                               787 568                                         High boilers                                                                            [% by wt.]                                                            DMT DMI DMO                                                                             [% by wt.]                                                            MMT      [% by wt.]                                                            TPA      [% by wt.]                                                            TAE      [% by wt.]                                                            p-TE     [% by wt.]                                                            p-TA     [% by wt.]                                                            BME      [% by wt.]                                                            Xylenes  [% by wt.]                                                            Methyl acetate                                                                          [% by wt.]                                                            Methyl formate                                                                          [% by wt.]                                                            Acetic acid                                                                             [% by wt.]                                                            DME      [% by wt.]                                                            Formic acid                                                                             [% by wt.]                                                            Acetaldehyde                                                                            [% by wt.]                                                            Methanol [% by wt.]                                                            Carbon dioxide                                                                          [% by wt.]                                                                           8,09                                                                               8,09                                                                               8,09                                                                               8,09                                                Oxygen   [% by wt.]                                                                           2,51                                                                               2,51                                                                               2,51                                                                               2,51                                                                               23,04                                                                              23,04                                       Carbon oxide                                                                            [% by wt.]                                                            Nitrogen [% by wt.]                                                                           86,77                                                                              86,77                                                                              86,77                                                                              86,77                                                                              76,27                                                                              76,56                                       Water    [% by wt.]                                                                           2,63                                                                               2,63                                                                               2,63                                                                               2,63                                                                               0,69                                                                               0,40                                        NO.sub.x as NO.sub.2                                                                    [% by wt.]                                                                           traces                                                                             traces                                                                             traces                                                                             traces                                              __________________________________________________________________________      Comment: traces <0,01% by weight                                               *Reference temperature 0° C.                                      

                  TABLE 2                                                          ______________________________________                                                        Energy use and distribution [kW]                                Energy-                            Electr.                                                                              Waste                                 stream Use           Steam    Fuel energy                                                                               heat                                  ______________________________________                                         200    Condenser 4                       7158                                  202    Pump 6                      2                                           204    Heat exchanger 8                                                                             -103                                                      206    Pump 9                      3                                           208    Pump 10                     1                                           210    Heat exchanger 11                 289                                   212    Pump 15                     4                                           214    Heat exchanger 17                 278                                   221    Intermediate heat                 6128                                         exchanger                                                               222    Heat of combustion     4940                                                    for DME                                                                 224    Heat of combustion     8150                                                    for DMT-residue                                                         226    Heat of combustion     5720                                                    for natural gas                                                         228    Heat exchanger 52                                                                            -11025                                                    230    Hot gas turbine 54          -8672                                       232    Air compressor 56           8672                                        ______________________________________                                          Reference temperature 0° C.                                       

Abbreviations

DME--Dimethyl ether

DMI--Dimethyl isophthalate

DMO--Dimethyl orthophthalate

DMT--Dimethyl terephthalate

MMT--Monomethyl terephthalate

p-TA--para-Toluic acid

p-TE--Methyl para-toluate (pT ester)

p-X--para-Xylene

TPA--Terephthalic acid

TAE--Terephthalaldehydic acid methyl ester

BME--Methyl benzoate

High boilers are organic substances which are not defined and which have boiling points higher than those of the above-mentioned substances. 

We claim:
 1. In a process for the purification of an off-gas stream originating from a process of oxidizing a liquid mixture of para-xylene with methyl para-toluate with an oxygen-containing gas under greater than atmospheric pressure, said off-gas being contaminated with at least one of an aliphatic and an aromatic impurity, which process comprises expanding said off-gas stream in an expansion turbine for generating mechanical or electrical energy so as to obtain an at least partial recovery of the energy contained in the off-gas, the improvement comprising subjecting said off-gas to at least one absorption step under said pressure to remove at least one combustible component from said off-gas and combusting the resultant off-gas stream under a pressure of more than 3 bar with the addition of oxygen without added fuel prior to said expanding of the off-gas stream in said expansion turbine.
 2. A process according to claim 1, wherein prior to said combustion, organic substances present in the off-gas stream are removed at least partially by condensation and subsequent absorption with an ester or ester mixture.
 3. A process according to claim 2, wherein the off-gas to be purified originates from an oxidation with air under a pressure of 5 to 50 bar, and said off-gas stream contains said para-xylene as a substance to be absorbed.
 4. A process according to claim 2, wherein said absorption is conducted with methyl para-toluate (p-TE) or a mixture of p-TE and methyl benzoate (BME).
 5. A process according to claim 2, wherein the ester or ester mixture used for absorption is obtained from a process step downstream of said oxidation process.
 6. A process according to claim 2, wherein said off-gas, after cooling and partial condensation of the organic substances dissolved therein, is purified in a first absorption stage with methyl para-toluate or a mixture of methyl para-toluate with methyl benzoate; in a second absorption stage, with methyl benzoate or methanol; and, in a third absorption stage, with water.
 7. A process according to claim 6, wherein a process waste water containing at least one member selected from the group consisting of methanol, acetic acid, formic acid, and formaldehyde is used as the wash water in the third absorption stage.
 8. A process according to claim 6, wherein the off-gas stream prior to combustion is saturated with a process waste water, said process waste water containing at least one member selected from the group consisting of methanol, acetic acid, formic acid, and formaldehyde, having been used as the wash water in the third absorption stage.
 9. A process according to claim 8, wherein prior to the saturation step, the off-gas is heated.
 10. A process according to claim 1, wherein the pressure in the combustion step corresponds approximately to the pressure in the oxidizing step.
 11. A process according to claim 1, wherein the combustion pressure is about 5-50 bar.
 12. A process according to claim 1, wherein the combustion pressure is more than 3 bar to about 7 bar.
 13. A process according to claim 1, wherein the combustion pressure is about 7 bar.
 14. A process according to claim 1, wherein the aliphatic and aromatic impurities within the off-gas are selected from the group consisting of para-xylene and methanol and are removed before combustion by partial condensation and subsequent absorption with an ester or ester mixture. 